This is a spool holder specifically designed for the AnyCubic i3 Mega-S. (And possibly the Mega Pro/Mega X?? .. Looks like they may use the same factory spool holder)
What this isn't: This isn't designed to be the smoothest rolling spool holder in the world. It doesn't have wheels or bearings or anything like that. I'm still kind of on the fence as to whether or not that stuff is even necessary. I've never really had problems related to that using the stock spool holder.
What this is: I was loaned a spool of AMZ3D filament, and found that the inner diameter of the spool was too small to fit on the stock spool holder. If you unscrew the post that the filament normally sits on, leaving the "arm" attached, this spool holder should attach to the existing factory arm and allow you to use spools with an inner diameter as small as about 30mm.
Things do fit together fairly snugly, and it may even be possible to use this without any screws (I didn't need them), but for extra insurance (or if it is looser for you due to printer calibration) I have included a screw hole to make sure that the square "attachment" piece stays on the arm, and another screw hole to make sure that the post stays on. You should be able to self-tap the factory screws that are normally used to attach the factory post to the arm in to the screw holes on this design. One screw goes in the left side (closest to your build plate), and one screw goes in to the top of the post once it is properly seated. You will notice that there is also a hole on the right hand side of the square piece. That one doesn't need to be used, and I only put it there in case someone wants to run a bolt all the way through with a nut on the other side. I think it is best to self tap the screws before putting it all together so that once you do put it together, you can just focus on making sure things are lined up correctly. A longer screw than stock would probably be preferable for the screw that goes in to the post from the top, if you have one.
Note that the metal hole that the screw is going through from the left is not threaded and doesn't need to be. Do not over tighten either screw. They don't need to be overly tight, and they will be fairly easy to strip if you aren't careful.
Be sure to check out my other Anycubic i3 Mega accessories!